When can you use a Discovery card or item, and does it cost an action?

Question

Accepted Answer

At any time during your own turn unless the card says otherwise, but never while resolving another card, effect, action or dice roll. Using an item is not an action unless the card says so, and spending Incantation tokens to cast a spell is free too.

Cthulhu: Death May Die

When can you use a Discovery card or item, and does it cost an action?